1. Field of the Invention
The present invention relates to a lead-acid battery separator.
2. Description of the Related Art
The method for producing a separator mainly used in a lead-acid battery is classified into two groups, namely, a method using an extrusion molding apparatus to form a sheet-like separator (U.S. Pat. Nos. 3,351,495 and 5,336,573), and a method using paper making process to form a sheet-like separator (U.S. Pat. No. 4,367,271). In the former, after ultra-high-molecular-weight polyethylene powder, silica and petroleum oil are heated, melted and kneaded and the mixture is extruded in the form of a sheet by an extrusion molding apparatus, a major part of petroleum oil is extracted with an organic solvent to obtain porosity to thereby form a so-called polyethylene separator. In the latter, binder is mixed with thermo plastic synthetic fiber and inorganic filler as main components and sheet-making is performed by paper making process to thereby form a so-called synthetic pulp separator. The synthetic pulp separator is generally stuck to a glass mat in use.
In recent years, the separator for a lead-acid battery, especially for a SLI battery has been required (1) to be lower in electrical resistance, (2) to have excellent oxidation resistance at a high temperature, (3) to prevent penetration and short-circuiting with an active material, (4) to have structures such as a more reliable shape (for example, the shape of a envelope), etc., because of a countermeasure against Products Liability and (5) to be inexpensive.
(1) As the separator low in electrical resistance, a separator more excellent in cold start characteristic of the battery has been required with the advance of down sizing of the battery.
(2) Oxidation resistance has been required not only with the increase in the frequency of use of the battery under a high temperature with the advance of reduction in space of underhood but also with the advance of motorization in tropical regions such as Southeast Asia, etc.
(3) With respect to the penetration and short-circuiting, electric power consumption due to the key off current in apparatuses with the increase of electric equipments, etc. has become to be hardly ignored. Accordingly, named hydrous short circuit has been actualized.
(4) A structure in which a sense of security is given by means of enveloping electrode plates or has a tendency to be welcomed with a favorable impression.
(5) The polyethylene separator requires a long time for petroleum oil extraction with an organic solvent and, further, a high expense is required for equipment and as a running cost because an apparatus for recovering the organic solvent and petroleum oil is needed. On the other hand, the synthetic pulp separator is made expensive because the synthetic pulp separator needs a stuck glass mat.
The polyethylene separator is apt to be oxidized because a network constituting pores is a polyethylene thin film. Particularly under a high temperature, deterioration is accelerated so that many cracks give rise in the separator, thereby causing to short the lifetime of the battery.
Further, it is generally considered that a separator having a smaller pore size is better to prevent the penetration of an active material causing short-circuiting. The aforementioned separator is however poor in diffusion of the electrolyte because the pores are too small. Accordingly, a stratification of electrolyte occurs to cause shortening of lifetime.
The polyethylene separator has the following defects. A great deal of care and a large equipment cost are required for prevention of the risk of a fire and for prevention of the pollution of the environment in a production process because of use of petroleum oil and an organic solvent. Further, the electrolyte in the battery will be contaminated with oil remaining in the separator, which oil is necessary for improvement of acid resistance of the separator.
In the synthetic pulp separator, the cumulative volume of pores having a pore size not smaller than 1 .mu.m is 40-60% of the cumulative volume of all pores. Because the pore size is too large to prevent the penetration of an active material, penetration short-circuiting occurs when discharge is deep. Further, to prolong the lifetime of the battery, the synthetic pulp separator must be generally used in combination with a glass mat. In that case, there is a defect that cold start characteristics can be obtained with much difficulty because evolved gas from electrodes will remain in the glass mat as bubbles (so-called bubble holding phenomenon).